The invention is based on a brushless direct-current drive with a synchronous motor and having a stator supporting a multi-phase stator winding and a rotor with permanent magnet poles for generating a magnetic flux that penetrate the stator winding.
In motor vehicles, permanent magnet-excited, brushless direct-current drives are used for a variety of purposes, including for electric steering boosters. These direct-current drives have a synchronous motor with a stator or armature winding and a permanent magnet-excited rotor. The armature winding is connected to the DC voltage network by means of an inverter in a bridge circuit with semiconductor power switches. The inverter that executes the commutation of the stator winding is triggered by an electronic control unit.
DE 37 09 168 A1 describes a synchronous motor operated in a DC voltage network in which three semiconductor power switches triggered by a control unit are disposed in the winding strands of the stator winding, which is wired in a star pattern. If malfunctions occur in the stator winding and/or in the power switches, then the direct-current drive can generate a strong electromagnetic braking moment without the application of a DC voltage since the synchronous motor then functions as a generator in opposition to a low-ohm load resistance. In many applications, such a braking moment impairs the function of the unit or system in which the direct-current drive is being used. Thus for example in electric steering boosters, the braking moment occurring in the event of a malfunction exerts considerable steering forces, which the driver must resist with his own physical strength in order to take the necessary countermeasures. There is thus the danger that the driver will no longer be able to steer the vehicle as desired and will lose control of the vehicle. It is known to provide these direct-current drives with devices, which in the event of such a malfunction, produce a so-called fail-silent behavior of the direct-current drive, i.e. the malfunctioning direct-current drive has no disruptive or disadvantageous influence on the unit or the system and the system therefore functions as if the direct-current drive were not present.
In a known electric steering booster, the desired fail-silent behavior is produced by means of a mechanical clutch via which the driven shaft of the synchronous motor engages the steering booster. In the event of a malfunction, the clutch disengages and consequently mechanically disconnects the motor from the steering system.
A hybrid-excited electrical machine is known (EP 0 729 217 B1), in which the magnetic field of the rotor is generated both by means of permanent magnets and by means of a field excitation winding, which is supplied with excitation current via rotor slip rings. The rotor is axially divided into two rotor halves, which are mounted spaced axially apart from each other on the rotor shaft. Receiving openings are provided in the lamination bundle of each rotor half and the permanent magnets are inserted into them. In terms of their polarity, the permanent magnets are disposed in the rotor halves so that in the one rotor half, they point toward the air gap of the machine with their north pole and in the other rotor half, they point toward the air gap of the machine with their south pole. The permanent magnets in the two rotor halves are offset in relation to one another by a pole division. The field excitation winding embodied in the form of an annular coil is inserted into the intermediary space between the two rotor halves. If the field excitation winding is supplied with DC voltage, then a magnetic flux is generated, which intensifies or weakens the magnetic flux of the permanent magnets depending on the flow direction of the excitation current. This yields a large regulating range for the speed and the voltage of the machine.
The brushless direct-current drive according to the invention, has the advantage that the desired fail-silent behavior of the direct-current drive is achieved through simple control actions in the direct-current drive itself, without costly external components of the kind represented by mechanical clutches. Through appropriate triggering of the field excitation winding, in the event of a malfunction, the magnetic field of the permanent magnets can be weakened or strengthened by means of the magnetic field of the field excitation winding so that no induced voltage or only a reduced one occurs in the synchronous motor and consequently, no short circuit current or only a reduced one can flow, which generates no braking moment or only a very slight one. In this connection, a moderate temperature-induced decrease in the magnetic field leads to a decrease in the braking moment and prevents an irreversible demagnetization of the permanent magnets and therefore prevents them from being permanently damaged. If a reduction in the braking moment is insufficient, then the magnetic field as a whole can be reduced to zero, but permanent damage to the permanent magnets must then be accepted.
If the field excitation winding of the rotor is supplied with current not only in the event of a malfunction, but also during normal operation, then on the one hand, an addition of the magnetic fields in the motor achieves a high power density and on the other hand in speed-variable operation, higher speeds can be achieved through a deliberately executed field weakening.
According to a preferred embodiment of the invention, the field excitation winding has a number of coils that corresponds to the number of permanent magnets and each coil is wound around one of the permanent magnets. The coils are connected in parallel or in series and are connected to a pair of rotor slip rings that are connected to the rotor shaft in a non-rotating fashion. This structural integration of the field excitation winding into the synchronous motor requires only a slight additional structural and production-related expense in order to produce the desired fail-silent behavior.